The present invention concerns the control of High-Intensity Discharge (“HID”) lamps. More specifically, the present invention concerns improvements to circuits and methods for enhanced control of HID lamps during the lamp warm-up phase.
HID lamps are currently broadly known and used in many different applications. HID lamps include, for example, mercury vapor lamps, high-pressure as well as low-pressure sodium vapor lamps, metal halide or ceramic metal halide lamps, and Xenon short-arc lamps.
Generally speaking, a HID lamp is a type of electrical lamp which produces light by means of an electric arc between electrodes housed inside a refractory and translucent or transparent housing constructed, for example, of fused quartz or fused alumina. Gas and metal salts are placed in the housing. After ignition of the lamp, gradual heating and evaporation of the metal salts forms a plasma in the refractory housing, which increases the light intensity of the lamp.
Following ignition, HID lamps require a warm-up phase, during which the lamp gradually heats until the electrodes reach a steady-state temperature. After ignition when the lamp is cold, the lamp voltage (the voltage across the electrodes of the lamp) is approximately 20V, while the steady-state voltage ranges approximately between 80-100 V.
To have efficient lamp management, it is necessary to have a warm-up phase of the lamp during the very first minutes after ignition. During warm-up, the lamp behaves as a resistor which changes resistance with the lamp temperature. The resistance increases with the temperature from a minimum value of approximately 6 ohms to a maximum value near 40 ohms. This behavior is linear or can be linearized between a start point and a stop point. Warm-up ends when the steady state voltage is reached. During the run-up phase, the lamp current must be limited to 1.2-1.5× the steady-state rated lamp current.
Typical HID ballasts are designed as shown in FIG. 1. The ballast includes an EMI filter and a rectifier (not shown), a booster, a buck converter and an H-bridge (full bridge) connected to the HID lamp electrodes. As shown in FIG. 1, the buck converter includes a current control circuit and a voltage control circuit, implemented with a DC-offset on the CS (current sense) pin of the PWM controller (L6562). The reason for adding the voltage control on the PWM controller is that a conventional buck converter using a simple current control would not allow reaching a required run-up current limit, and during the thermionic emission the current would increase beyond 1.2-1.5× the steady-state rated lamp current. The voltage control circuit connected to the CS pin of the PWM controller modifies the control circuit from a simple current control to a mixed “current and voltage control”. During run-up, the buck current will take thus the form shown in FIG. 2. This results in a sufficient current stability during all run up phases between 20V up to 100V lamp voltage.
An electronic ballast circuit of this kind is complex and expensive. Specifically, analog circuitry to manage the warm-up phase in a typical HID ballast circuit is prone to failure and increases the cost of the circuit. Additionally, the warm-up current cannot be controlled in a reproducible manner, because it varies depending upon the lamp technology and lamp supplier, i.e. the same ballast provides different warm-up current values for different lamps. Moreover, the duration of the warm-up phase is not optimized.
Additionally, analog circuits for current control during warm-up suffer from intrinsic limitations due the manner in which the current signal is generated. As can be clearly seen in FIG. 1, the current signal is obtained by means of a current sensor (Rsense) arranged on the return line of the buck converter. Particularly during the warm-up transitional phase, the current circulating in the buck converter does not correspond to the actual current across the lamp and is variable in time as a consequence of the variable temperature conditions of the lamp. Complex measures must be implemented to correct the current sensed via the sensor resistor Rsense, so as to remove therefrom the amount of current which is circulating in the full bridge but not across the HID lamp.
Additionally, the waveform of the current circulating in the buck converter is complex and this adds to the difficulty in obtaining a current signal usable in a control loop.
Two-stage booster-buck ballasts are not the only possible devices suitable for powering HID lamps. A single-stage HID ballast is disclosed in U.S. Pat. No. 7,190,151. An HID ballast with glow arc and warm-up control is disclosed in US Patent Publication 20030222596. Similar problems as those discussed above in connection with the topology of FIG. 2 are encountered also in prior art ballasts of different kinds.
FIG. 1A shows a block-diagram of a conventional warm-up control circuit using a digital microcontroller. The HID lamp control hardware (a one or two stage ballast) provides a lamp-current signal (I) and a lamp-voltage signal (V). The lamp-voltage signal is used as the feedback signal in the control loop. A correction value is added to the current feedback to generate a corrected feedback signal (I′feedback). Such corrected feedback signal is compared in a control loop to a fixed current reference (IREF). The correction loop generates a PWM signal to drive the HID hardware, such that the warm-up current is maintained substantially at a constant value during warm-up.
Usually a microcontroller is used to sample a lamp voltage. The sampled and digitized voltage signal is used as an index for a look-up table that contains the correction values to be added to the current feedback depending upon the actual lamp voltage. An analog correction circuit can be used, driven by corrected feedback value, or alternatively an entirely digital correction circuit is used.
Analog correction circuits and microcontroller look-up tables need to be manually adapted to work correctly and to be kept at the desired warm-up lamp current. This means that the analog circuit and/or the look-up table must be adapted to the lamp model, i.e. it is dependent upon the lamp technology, the lamp manufacturer and so on.